text: HDS systems use the physics of flowing water to remove a variety of pollutants and are characterized by an internal structure that either creates a swirling vortex or plunges the water into the main sump. Along with supplemental features to reduce velocity, an HDS system is designed to separate floatables (trash, debris and oil) and settleable particles, like sediment, from stormwater. HDS systems are not effective for the removal of very fine solids or dissolved pollutants. The systems are also subject to scour and sediment washout during large storm events, e.g. a 10-year storm.